Coherent beam device for observing and measuring sample

ABSTRACT

In an interferometric device using a coherent beam and an image pickup camera, a protective glass for the detector surface is needed on the incident side of the pickup unit of the image pickup camera, and this gives rise to interference fringes which constitute noise. To solve this problem, a dustproof container is configured also to cover an imaging lens system arranged on the incident side of the pickup unit of the image pickup camera, and the pickup unit of the image pickup camera is arranged in this container. By assigning the function of the protective glass for the detector surface in the conventional configuration to the imaging lens system, the protective glass is made unnecessary.

BACKGROUND OF THE INVENTION

The present invention relates to a device for observing or measuring theimage of a sample by using a coherent beam, and more particularly to aninterferometric device suitable for precise measurement of a coherentbeam having been transmitted through or reflected by a sample.

Such a device according to the prior art would cause a laser beamtransmitted or reflected by a sample to directly form an image in acamera by-using an imaging lens, as does the laser interferometerillustrated in Optical Shop Testing, Chapter 14 (Reference 1, J. E.Greivenkamp and J. H. Bruning, Chapter 14 “Phase ShiftingInterferometer” in D. Malacara, ed., Optical Shop Testing, SecondEdition, 1992, John Wiley &amp; Sons, Inc.) FIG. 14.2,(a)-(c). Forinstance in the example illustrated in FIG. 14.2(b), a beam emitted froma laser is enlarged by a beam expander into a plane wave having a largediameter, and divided into a beam transmitted through a firsttranslucent mirror and a beam reflected by it. The transmitted beam,after having penetrating a sample, is altered upward in optical path bya reflector, reflected by a second translucent mirror, and is caused byan imaging lens to form an image on the detector surface of the camera.The beam reflected by the first translucent mirror is changed rightwardin optical path by the reflector, transmitted through the secondtranslucent mirror, and is caused by the imaging lens to form an imageon the detector surface of the camera. On the detector surface-of-thecamera, the beam which has been transmitted through the sample andundergone a change in wavefront from a plane (i.e. the object wave) andthe plane wave to be referenced (i.e. the reference wave) interfere witheach other to form an image consisting of interference fringes.

In this interference image is preserved the distribution of phases ofthe beam transmitted through the sample. This distribution of phases canbe measured by one of several methods. The most basic one is a method bywhich the directions of the object wave and of the reference wave arebrought to coincidence by controlling the inclination of the secondtranslucent mirror to obtain the contour lines of the interferencefringes of the object wave. In this case, as a contour line emergesevery time the phase change reaches one wavelength, there is no problemwhen the phase change is substantial, but a phase change less than onewavelength needs to be discerned by the relative shade.

One of the methods for precise measurement of this less than onewavelength phase change is the phase shift method. By this method, thedistribution of phases attributable to the sample preserved in theobject wave is figured out by calculation from three or moreinterference images that have been taken in while controlling therelative phase-difference between the object wave and the referencewave. As shown in FIG. 14.2(b), one (usually on the reference beam side)of the reflectors is mounted on a movable stage driven by a piezoelement and, if it finely moves in the direction of a line normal to thereflector for instance, though the phasic state of the object wave onthe observation plane does not change, that of the reference wave on theobservation plane does change because the optical path length varies,with the result that only the interference fringes move while the imageof the sample remains unmoved. To take note of a certain point, with avariation of the reference wave in optical path length, the brightnessof that point varies according to a sine curve. The quantityrepresenting the starting position of this sine curve on a phasic basisis the phase of the object wave at that point. Therefore, by figuringout the sine curve at each point in the field of view, the distributionof phases attributable to the sample recorded on the object wave can befound.

Although the case described above refers to the transmission of a beamby the sample, an interferometer of the type using a beam reflected bythe sample for measurement as shown in (a) and (c) of FIG. 14.2 inReference 1 is based on exactly the same method and principle ofmeasurement.

SUMMARY OF THE INVENTION

In the interferometric method described in the foregoing section, anerrorvin measurement could arise from:

-   -   1. Mechanical vibration propagating to the interferometer;    -   2. Oscillation of air on the optical path;    -   3. Accuracy of piezo driving;    -   4. Accuracy of the reflecting faces of the reflector and of the        translucent mirror in planarity and thickness uniformity;    -   5. Stability of the frequency and strength of the laser;    -   6. Linearity of the camera with respect to any distortion of the        picked-up image and the strength of the image output;    -   7. Accuracy of calculation;    -   8. Noise on the image signal line, or    -   9. Interference fringes due to front and back face reflections        at the translucent mirror, the lens and the camera. Of these        factors, those cited in 1, 3, 4, 8 and 9 are more influential        than others, and particularly that in 9, which is based on the        very principle of the method, is difficult to reduce.

The present invention has been attempted to reduce the influence ofinterference fringes due to front and back face reflections at theentrance surface of the detector part in the camera out of these adversefactors.

In front of the detector surface of a TV camera, if it is a camera tube,there is a glass for keeping vacuum and transmitting a beam or, if it isa solid state imaging camera, there is a protective film or a protectiveglass for protecting the surface of the image pickup element. As theprotective film or the protective glass differs from air in refractiveindex, reflected beams arise on the incident face and the emitting face.A beam reflected by the emitting face gives rise to another reflectedbeam on the incident face, whose interference with the incident beamresults in superposition of interference fringes on theobserved/measured image. How a beam coming incident on such a protectiveglass is reflected is illustrated in FIGS. 1(a) and 1(b).

While a greater part of the incident beam represented by a solid line inFIG. 1(a) is transmitted through the incident face of a protective glass100 as a transmitted beam 101, primary reflected beams arise on theincident face and the emitting face. The primary reflected beam on theemitting face is represented by a broken line 102, and the illustrationof the primary reflected beam on the incident face is dispensed withbecause it is irrelevant to the explanation of the principle. Thisprimary reflected beam 102 further gives rise to a secondary reflectedbeam 103 on the incident face. This is represented by a one-dot chainline. A greater part of the primary reflected beam 102 is transmitted,though not shown here. A greater part of the secondary reflected beam103 is transmitted through the emitting face, again only partiallyreflected. The resultant tertiary reflected beam is not illustratedeither. The three beams shown here are actually on the same line, butexpressed at vertically different levels for the convenience ofillustration.

Although the incident face and the emitting face of the protective glass100 are depicted in FIG. 1(a) as exactly parallel planes, actually thereare always infinitesimal unevenness. If they are exactly parallelplanes, the brightness will be even all over as a result of interferencebetween the transmitted beam 101 and the secondary reflected beam 103,but if there is unevenness, interference fringes will be formed. Thesestates are illustrated in FIG. 1(b). Three vertical lines 115 on theleft represent the wavefront of the incident beam 101, three verticallines 116 on the right, the wavefront of the transmitted beam 101, andthree corrugated lines 117, the wavefront of the secondary reflectedbeam 103. Variations in wavefront shape attributable to the refractiveindex-of the protective glass 100 are disregarded because they aresimple. Further, the unevenness of only the incident face is considered,but variations of the uneven wavefront shape with the procession of thebeam is disregarded.

As illustrated, the transmitted beam 101 is unaffected by anydeformation of the wavefront and retains its plane wave form. Theprimary reflected beam 102, though it also is a plane wave because it isreflected by a plane, is not illustrated. The secondary reflected beam103, as it is reflected by an uneven face, becomes a wavefront whoseunevenness is double that of the incident face. The interference fringesresulting from interference between the transmitted beam 101 and thesecondary reflected beam 103 are brighter where wavefronts overlap eachother and darker where there is a lag of half interval betweenwavefronts. How this occurs is shown in FIG. 1(c).

This phenomenon can occur between the emitting face of the protectiveglass 100 and the photoelectric conversion film of the camera tube orthe surface of the solid image pickup element. Thus in the conventionalimage observing or measuring device using a coherent beam, the glass orprotective film in front of the detector surface of the camera givesrise to interference fringes, which deteriorate the image quality in theimage observing device and also adversely affects the measuring accuracyof the interferometric device.

A number of solutions have already been proposed to this problem. Forinstance, the Japanese Patent Application Laid-open No. Hei 5-316284“Image Pickup Device with Preventive Mechanism Against Noise ofInterference Fringes” proposes to incline the protective glass, theJapanese Patent Application Laid-open No. Hei 8-145619 “LaserInterferometer”, to-shape the protective glass like a wedge, and theJapanese Patent Application Laid-open No. Hei 8-191418 “Image PickupDevice with Preventive Mechanism Against Noise of Interference Fringes”,a planoconvex lens as the protective glass, but all these ideaspresuppose the indispensability of a protective-glass in front of thedetector surface and consider the best way to provide one.

One of the causes for a deterioration in image quality or a drop in themeasuring accuracy of interferometry in the conventional image observingor measuring device using a coherent beam is, as noted above, theinterference fringes due to front and back face reflections of thetransparent planar member, such as a protective glass, arranged in frontof the image pickup device of the imaging apparatus. A conceivable wayto prevent it is to coat the front and back faces of the transparentplanar member against reflection, matched with the wavelength of thecoherent beam to be used. However, though it is possible to reduce thereflections by 1 or 2%, this is insufficient for highly preciseinterferometry.

This problem can be intrinsically solved by doing away with thetransparent planar member. In a camera tube type device, this member isdifficult to remove because it constitutes part of a vacuum container,but in a solid image pickup element it can be done away with because themember is intended merely for surface protection. In this case, as theremoval would result in exposure of the surface of the solid imagepickup element, any smear or dust sticking to that surface would bedifficult to remove, involving many problems including the fear ofdamaging the surface anew in a removing attempt. A possible way toeliminate this fear is to work out a solid state imaging camera in whichonly those parts integrated with the solid image pickup element can besimply replaced.

Alternatively, if the whole coherent beam device is housed in adustproof-structured container, the possibility of dust or smear beingstuck to the surface of the solid image pickup element can besubstantially reduced. However, this structure would not only require alarger container but also involve awkwardness in the operation toreplace the sample, making it necessary to open the container at leastpartly and entailing a greater fear of letting minute dust to enter intothe container together with the sample.

It is therefore proposed to take note of the combination of theconstituent elements of a coherent beam device and a solid state imagingcamera, and to provide a dustproof-structured container in which areaccommodated, out of optical components in positions not affecting theoperation to replace the sample, such as the imaging lens andinterference elements, the elements-whose positions are fixed inobserving or measuring an image are arranged on the incident side, andcomponents integrated with the solid state imaging camera or the solidimage pickup element of the solid state imaging camera.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) illustrates how reflections on the front and back faces of aprotective glass take place when the incident face and the emitting faceof the protective glass of the focal plane of the image pickup cameraare exactly parallel planes; FIG. 1(b), how interference fringes due toreflections on-the front and back-faces of the protective glass arisewhen the incident face and the emitting face of the protective glass ofthe detector surface of the image pickup camera are not exactly parallelplanes; and FIG. 1(c), interference fringes arising from interferencebetween a beam transmitted through the protective glass and a secondaryreflected beam.

FIG. 2 shows a first preferred embodiment of the present invention.

FIG. 3 shows a second preferred embodiment of the invention.

FIG. 4 shows a third preferred embodiment of the invention.

FIG. 5 shows a fourth preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment I

A first preferred embodiment of the invention is shown in FIG. 2. Thisinterference optical system causes a reflected beam from a concavemirror and a reflected beam from a reference mirror to interfere witheach other, and checks any distortion of the concave mirror from theshape of interference fringes. A laser beam 20 emitted from a laser 1 isshaped into a parallel radiating beam having a large diameter with acollimator lens system 2, and this beam is split by a beam splitter 3,which is a translucent mirror, into a transmitted beam 21 and areflected beam 22. The reflected beam 22 is reflected by a highly planarreference surface 8 to become a reference beam 23 (the part hatched withleftward slopes) and, after being transmitted through the beam splitter3, is projected by an imaging lens 6 on the detector surface of a solidstate imaging camera 17. On the other hand, the transmitted beam 21 isturned into a divergent light centering on a focus 9 by a collectivelens 9, and this divergent light irradiates a checked surface 5. Thechecked surface 5 is fabricated in a spherical face centering on thefocus 9. If the checked surface 5 is an ideal spherical face, the parthatched with rightward slopes reflected by the checked surface 5 willconstitute a spherical wave converging on the focus 9, and be convertedby the collective lens 9 again into a parallel beam, i.e. a plane wave.This checked surface-reflected beam 24, after being reflected by thebeam splitter 3, is projected by the imaging lens 6 on the detectorsurface of the solid state imaging camera 17. The solid state imagingcamera 17 is supplied with power and controlled by a solid state imagingcamera power source 18, and its image is displayed on an image observingmonitor 19. In the overlapping part (the cross-hatched part) of thereference beam 23 and the checked surface-reflected beam 24, as-twoplane waves overlap each other, there are formed interference fringes,and the image of the solid state imaging camera 17 consists of linearinterference fringes. When the directions of the two plane waves aremade identical by adjusting the inclination of the beam splitter 3,there is achieved a state of uniform brightness all over.

However, actually the checked surface 5 slightly deviates from a sphereon account of the limitation of working accuracy. Since the machining ofa sphere is usually less precise than that of a plane, the deviation ofthe checked surface 5 from a sphere is greater than that of thereference surface 8 from an ideal plane. Therefore, the interferencefringes arising from the interference between the reference beam 23,which can be deemed to be a plane wave, and the checkedsurface-reflected beam 24 deviating from a plane wave correspondingly tothe deviation of the checked surface 5 from a-sphere are off a straightline according to the location and magnitude of the deviation of thechecked surface 5 from a sphere, and it can be determined whether or notthe checked surface 5 is within a certain standard range by bringing thedirections of the two beams to coincidence.

In a conventional interferometer, as a protective glass is adhered infront of the detector surface of the solid state imaging camera,interference fringes due to reflections on the front and back faces ofthe protective glass superpose over the measured image as described withreference to FIG. 1, and they cannot be distinguished by thedistribution of shades due to the deviation of the checked surface 5from a sphere, often posing obstacles to accurate checking. In view ofthis problem, for this embodiment of the invention, a solid stateimaging camera with no protective glass in front of the detector surfacehas been produced, and the detector surface is positioned to beidentical with the observation plane of the interferometric system.Since the absence of a protective glass exposes the detector surface tothe risk of damage due to the adhesion of dust, the solid state imagingcamera 17 is housed within a dustproof container 10 of which one endconsists of the imaging lens 6. This has made it possible to do awaywith interference fringes due to reflections on the front and back facesof the protective-glass in the conventional configuration and to preventthe surface of the image pickup element from catching dust or beingsmeared. In this embodiment, the imaging lens 6, which would be one ofthe constituent parts of a conventional interferometer, is assigned theadditional role to serve as the protective glass for the solid imagepickup element.

Embodiment II

FIG. 3 shows a second preferred embodiment of the invention. Thisinterference optical system is intended to cause a beam transmittedthrough a sample and a reference beam to interfere with each other, andto measure the distribution of refractive indices within the sample fromthe shape of interference fringes. The laser beam 20 emitted from thelaser 1 is converted by the collimator lens system 2 into a parallelradiating beam 20 having a large diameter, and is split by a firsttranslucent mirror 11 into a reflected beam 41 (the part hatched withrightward slopes) and a transmitted beam 42 (the part hatched withleftward slopes). The reflected beam 41 is reflected by a firstreflector 13 to irradiate a sample 4. A sampled-transmitted beam 25 (thegray part) locally varied by the distribution of refractive indiceswithin the sample is transmitted through a second translucent mirror 12,and is caused by the imaging lens 6 to form an image on the detectorsurface of the solid state imaging camera 17. The transmitted beam 42 isreflected by a second reflector 14, further reflected by the secondtranslucent mirror 12, and is caused by the imaging lens 6 to form animage on the detector surface of the solid state imaging camera 17. Inthis case, as in the first preferred embodiment, highly planar elementsare used as the first translucent mirror 11, the second translucentmirror 12, the first reflector 13 and the second reflector 14, thetransmitted beam 42 is equivalent to the reference beam 23 in the firstembodiment, which can be deemed to be a plane wave, thesample-transmitted beam 25 deviates from a plane wave only reflectingthe refractive index within the sample 4, and these two beams interferewith each other (the cross-hatched part) to form interference fringes.When the directions of the two beams are brought to coincidence byadjusting the inclination of the second translucent mirror 12, therewill emerge a distribution of shades corresponding to the quantity ofphase variations attributable to the sample 4. This interference imageis displayed on the image observing monitor 19 via the solid stateimaging camera 17 and the solid state imaging camera power source.18 toenable the internal structure of the sample to be observed.

Since this embodiment of the invention, like the first embodiment, canalso have the imaging lens 6, which could be a constituent part of aconventional interferometer, perform the additional role to serve as theprotective glass for the solid image pickup element by housing the solidstate imaging camera 17 within the dustproof container 10, there is noneed to provide a protective glass in front of the detector surface ofthe solid state imaging camera, and it is free from the problem ofsuperposition of interference fringes due to reflections on the frontand back faces of the protective glass over the measured image.

Embodiment III

FIG. 4 shows a third preferred embodiment of the invention. Thisinterference optical system is intended to apply a high precisioninterferometric method known as the phase shift method to cause a beamreflected by a checked plane and a reference beam to interfere with eachother and thereby to measure the planarity of the checked plane. Thelaser beam 20 emitted from the laser 1 is shaped into a parallel beam 20having a large diameter with the collimator lens system 2, and split bya cubic beam splitter 15 into a transmitted beam 21 (the part hatchedwith rightward slopes) and a reflected beam 22 (the part hatched withleftward slopes). The reflected beam 22 is reflected by the checkedsurface 5, transmitted through the cubic beam splitter 15 as the checkedsurface-reflected beam 24, and caused by the imaging lens 6 to form animage on the detector surface of the solid state imaging camera 17. Thetransmitted beam 21 is reflected by the reference surface 8, furtherreflected by the cubic beam splitter 15 as the reference beam 23, andcaused by the imaging lens 6 to form an image on the detector surface ofthe solid state imaging camera 17. As the reference surface 8 is highlyplanar, the reference beam 23 becomes a beam which can be deemed to be aplane wave, and interferes with the checked surface-reflected beam 24from the checked surface 5 to form interference fringes (thecross-hatched part). When the directions of the two beams are brought tocoincidence by adjusting the inclination of the cubic beam splitter 15,there will emerge a distribution of shades corresponding to the quantityof phase variations according to the planarity of the checked surface 5to enable this planarity of the checked surface 5 to be evaluated.

In this embodiment, since the solid state imaging camera 17 is housedwithin the dustproof container 10 of which one end consists of theemitting face of the cubic beam splitter 15, no protective glass isrequired, and the configuration ensures the absence of interferencefringes due to reflections on the front and back faces of the protectiveglass. Further the possibility of the imaging lens 6 to catch dust issubstantially reduced by its being housed within the dustproof container10, making possible more precise measurement.

The difference of this embodiment from embodiments I and III consists inthe use of the phase shift method, whose basics are described below. Therelative phase-difference between a reference beam and a checked beam,or the checked surface-reflected beam 24 in this embodiment, is variedat a time by 1/M (M is a positive number of not smaller than 3) of thewavelength of the laser beam that is used, and the two-dimensional phasedistribution recorded on the checked beam is figured out by calculationeach time from the M interference images that have been taken in. Torealize this process, the reference surface 8 is fixed to a piezo-drivenstage 26, and is shifted by a prescribed infinitesimal quantity from acontrol/analysis computer 30 to an equipment control board 32 in thedirection of an arrow in the drawing to enable the optical path lengthof the reference beam 23 to be varied. An image from the solid stateimaging camera 17 is displayed on the image observing monitor 19 via thesolid state imaging camera power source 18, at the same time taken intothe control/analysis computer 30 via an image take-in board 31, andrecorded in an internal memory or some other storage device. Computationsoftware based on the phase shift method is built into thecontrol/analysis computer 30, and the distribution of the checkedsurface-reflected beam 24 figured out from the interference image thathas been taken in is displayed on a computer-serving monitor 33.

The interferometric system using the phase shift method permits far moreaccurate measurement than the usual interferometer proposed as the firstor second embodiment. According to the prior art, however, there is aheavy constraint on the accuracy of measurement because phase variationsdue to interference fringes resulting from reflections on the front andback faces of the protective glass of the image pickup camera aresuperposed over the measured results and they cannot be separated fromeach other. In this embodiment, as the solid state imaging camera 17having no protective glass for its detector surface is used as the imagepickup camera to prevent interference fringes due to reflections on thefront and back faces from occurring and housed together with the imaginglens 6 in the dustproof container 10 of which one end consists of thecubic beam splitter 15, the aforementioned constraint on the accuracy ofmeasurement is substantially eased.

It goes without saying-that, even if some other fine adjustment stage,such as a stepping motor-driven stage, is used here for finely movingthe reference surface 8 in place of the piezo-driven stage 26 or theimage observing monitor 19 and the computer-serving monitor 33 are usedin combination, similar effects and functions can be achieved. Of evenif the configuration does not use the phase shift method, asinterference fringes due to reflections on the front and back faces ofthe protective glass of the solid state imaging camera 17 do not arise,the accuracy will be higher than what the conventional method canprovide as is the case with the first two embodiments.

In an interferometer of this type, if the cubic beam splitter 15 isturned by an infinitesimal degree around an axis normal to the face ofthe drawing instead of finely shifting the reference surface 8 in thedirection of the optical axis, the phase shift method can beimplemented. In this case, since the imaging lens 6 and the solid stateimaging camera 17 need to be fixed to the interferometer during themeasurement process, the structure would be such that one end of thedustproof container 10 is the imaging lens 6 as in the first twoembodiments.

Embodiment IV

A fourth preferred embodiment of the present invention is shown in FIG.5. This interference optical system is another example of measuring thedistribution of refractive indices within a transmissive sample by usingthe phase shift method. In this drawing, neither the laser nor thecollimator lens system is shown. The parallel laser beam 20 irradiatesthe sample 4 placed on one side of the optical path, or in the lowerhalf of the drawing. The beam on the side where the sample is present(the part hatched with rightward slopes) serving as thesample-transmitted beam 25 and that on the side where the sample isabsent (the part hatched with leftward slopes) serving as the referencebeam 23, they are magnified at two stages by the imaging lens 6 and amagnifying lens 6′, and form an image on the detector surface of thesolid state imaging camera 17. Between the magnifying-lens 6′ and thesolid state imaging camera 17, there is provided a prismatic beamsplitter 16 mounted on the piezo-driven stage 26. The sample-transmittedbeam 25 and the reference beam 23 are caused by the prismatic beamsplitter 16 to overlap each other as represented by the cross-hatchedpart in the drawing, and form an image consisting of a group ofinterference fringes off a straight line according to the distributionof refractive indices within the sample.

To implement the phase shift method, the piezo-driven stage 26 mountedwith the prismatic beam splitter 16 is shifted by an prescribedinfinitesimal quantity from the control/analysis computer 30 via theequipment control board 32 as indicated by an arrow in the drawing. If,for instance, it is finely shifted upward, the phase of the transmittedbeam 25 is advanced uniformly as this beam passes a thinner part of theprismatic beam splitter 16 while the phase of the reference beam 23 isdelayed uniformly as this beam passes a thicker part. Therefore,interference fringes deviate upward, but M interference images are takeninto the control/analysis computer 30 via the image take-in board 31while having the prismatic beam splitter 16 finely shift thepiezo-driven stage 26 by 1/M (M is a positive number of not smaller than3) of interference fringes at a time, and recorded in an internal memoryor some other storage device. As in the third embodiment, thedistribution of the sampled-transmitted beam 25 is figured out from theinterference image that has been taken in by using computation softwarebuilt into the control/analysis computer 30, and is displayed on acomputer-serving monitor 33.

In this embodiment, since the solid state imaging camera 17, togetherwith the piezo-driven stage 26 on which the prismatic beam splitter 16is mounted, is housed within the dustproof container 10 of which one endconsists of the magnifying lens 6′, no protective glass is required andaccordingly interference fringes due to reflections on the front andback faces of the protective glass can be prevented from arising.Furthermore in this embodiment, not only the detector surface of thesolid state imaging camera 17 but also the prismatic beam splitter 16and one face of the magnifying lens 6′ can be prevented from catchingsmear. Although the prismatic beam splitter 16 shifts during themeasurement process, as the whole piezo-driven stage 26 is housed in thedustproof container 10, it is sufficient to lead only the electricalwiring of the piezo-driven stage 26 into the dustproof container 10 in adustproof way, and shifting of the prismatic beam splitter 16 poses noproblem. Also, this embodiment can be further developed into a dustproofcontainer 10 of which one end consists of the imaging lens 6, one faceof the imaging lens 6 and the magnifying lens 6′ can be prevented fromcatching dust, resulting in an even higher level of effectiveness.

In any of the embodiments described above, reflections on the front andback faces of the imaging lens give rise to interference fringes. Theyare intrinsically difficult to eliminate, and there is no otheralternative than to reduce them by anti-reflection coating.

The device for observing or measuring an image by using a coherent beamaccording to the present invention provides the advantage of improvingthe accuracy of observation and measurement as it can eliminateinterference fringes reflections on the front and back faces of theimage pickup element and further can prevent the image pickup element orsome of other optical components from catching dust or smear by housingthem in a dustproof container.

1. A coherent beam device for observing or measuring a sample wherein acoherent beam source, a checked sample, an optical path for letting passan object wave resulting from the wavefront variation of a beam fromsaid coherent beam source from a plane by said checked sample, anotheroptical path for letting pass a reference wave which references the beamfrom said coherent beam source, an imaging lens system for forming on anobservation plane an image consisting of interference fringes by causingsaid object wave and said reference wave to interfere with each other, asolid state imaging camera so placed that the surface of its exposedimage pickup element come to the position of observation plane, and thesolid state imaging camera and one periphery of the elements of saidimaging lens system are enclosed in a sealed container.
 2. The coherentbeam device, as set forth in claim 1, wherein said one periphery of theelements of the imaging lens system is the periphery of the imaging lensof said imaging lens system.
 3. The coherent beam device, as set forthin claim 1, wherein said imaging lens system has a cubic beam splitterand one periphery of the elements of said imaging lens system is theperiphery of one face of said cubic beam splitter or consists of fourfaces in contact with that one face.
 4. The coherent beam device, as setforth in any of claims 1 through 3, wherein the object wave resultingfrom the wavefront variation of the beam from said coherent beam sourceby said checked sample is the result of transmission of said beam bysaid checked sample.
 5. The coherent beam device, as set forth in any ofclaims 1 through 3; wherein the object wave resulting from the wavefrontvariation of the beam from said coherent beam source by said checkedsample is the result of reflection of said beam by said checked sample.6. The coherent beam device, as set forth in claim 5, wherein saidobject wave as the result of reflection by said checked sample isgenerated by a reflective face made finely shiftable in the direction ofthe beam from said coherent beam source for the purpose of imageanalysis by a phase shift method.
 7. A coherent beam device forobserving or measuring a sample wherein a coherent beam source, achecked sample, an optical path for letting pass an object waveresulting from the wavefront variation of a beam from said coherent beamsource from a plane by being transmitted through said checked sample,another optical path for letting pass a reference wave which referencesthe beam from said coherent beam source, a magnifying lens formagnifying said object wave and said reference wave, a prismatic beamsplitter for forming on an observation plane an image consisting ofinterference fringes by causing said object wave having been transmittedthrough the magnifying lens and said reference wave to interfere witheach other, a solid state imaging camera so placed that the surface ofits exposed image pickup element come to the position of the observationplane, and the solid state imaging camera and the periphery of saidmagnifying lens are enclosed in a sealed container.
 8. The coherent beamdevice, as set forth in claim 7, wherein said prismatic beam splitter ismade finely shiftable in the direction orthogonal to the beam from saidcoherent beam source for the purpose of image analysis by a phase shiftmethod.